Fine control for the preparation of ceria nanorods (111)

The morphologies and exposed surfaces of ceria nanocrystals are important factors in determining their performance. In order to establish a structure–property relationship for ceria nanomaterials, it is essential to have materials with well-defined morphologies and specific exposed facets. This is also crucial for acquiring high resolution 17O solid-state NMR spectra. In this study, we explore the synthesis conditions for preparing CeO2 nanorods with exposed (111) facets. The effects of alkali concentration, hydrothermal temperature and time, cerium source and oxidation agent are investigated and optimal synthesis conditions are found. The resulting CeO2 nanorods show very narrow 17O NMR peaks for the oxygen ions in the first, second and third layers, providing a foundation for future research on mechanisms involving ceria materials using 17O solid-state NMR spectroscopy.


Introduction
CeO 2 , a rare-earth metal oxide with cubic uorite structure, has many attractive properties and is critical in the chemical industry. [1][2][3][4][5] It is particularly useful for providing oxygen in oxygen-decient environments, generating nonstoichiometric oxide CeO 2−x , while this reduced oxide can store oxygen under oxygen-rich conditions. 6 Therefore, CeO 2 nds multiple applications in redox catalysis, including as three-way catalysts (TWCs), 7 in the water gas conversion reaction (WGS), 8 oxidation of volatile organic matter, 9 hydrogen purication, 10 petroleum cracking, 11 CO 2 hydrogenation, 12 and as a catalyst support. 13 Its excellent properties can be ascribed to its special geometric structure 14 and electronic structure, 15 as well as the low activation energy barrier for generating lattice oxygen vacancies. Nanomaterials, which have attracted a lot of research attention recently, are oen associated with better catalytic properties. It has been found that the morphologies 16 and exposed facets of nanocrystals play a crucial role in controlling the catalytic activity and selectivity. [17][18][19][20] In order to explore the relationship between structure and catalytic properties in detail, nanomaterials with well-dened morphologies and specic exposed facets are required. Many attempts have been made to prepare CeO 2 nanocrystals with specic morphology and facets, [21][22][23] and it is oen necessary to control the synthesis conditions, including alkali concentration, hydrothermal temperature and anions in the cerium source. 24,25 We have recently developed a new method based on 17 O solid-state NMR spectroscopy for distinguishing oxygen ions in different surface layers or different facets in oxide nanomaterials according to the chemical shi. 26,27 This method can be used to explore the detailed reaction mechanisms on these materials. The linewidths of the signals are dependent on the distribution of the local environments, such as bond angles and bond length. In order to obtain high resolution data and detailed structural information, the peak widths should be minimized. Therefore, CeO 2 nanocrystals with specic morphology and facets are required. In this paper, we explore the template-free hydrothermal synthesis of CeO 2 nanomaterials, and prepare CeO 2 nanorods exposing mainly (111) facets by controlling the alkaline solution concentration, hydrothermal temperature and time, cerium source and oxidation agent. We show that the CeO 2 nanorods prepared under optimized conditions exhibit very narrow linewidths in the 17 O NMR spectrum. and then transferred to a hydrothermal reactor for heating at 100 to 180°C for 24 h. Aer the hydrothermal treatment, the product was ltered, washed with deionized water and ethanol until a neutral pH was obtained, and then heated in an oven at 80°C for 3 h. Finally, the product was calcined at 700°C for 3 h under an air atmosphere, with the calcination temperature determined using thermogravimetric analysis (TGA) data ( Fig. S1 †). Further details on the preparation procedures are discussed in the Results and discussion section.
Characterization X-ray diffraction (XRD) was carried out on a Philips X'pert Pro diffractometer using Ka radiation from a Cu target (l = 0.15418 nm) with a Ni lter. The operating current and voltage were 40 mA and 40 kV, respectively. The scanning range of 2q was from 5°to 90°. Transmission electron microscopy (TEM) images of samples were taken on a JEOL-JEM-2010 transmission electron microscope operating at 100 kV. The TGA was carried out on a NETZSCH STA 449C, from room temperature to 700°C. The BET surface area and pore size distribution were determined from nitrogen isotherm at 77 K on a Micromeritics TriStar II 3020 instrument. X-ray photoelectron spectroscopy (XPS) was performed using a PHI 5000 Versa Probe manufactured by ULVAC-PHI, Japan. 17

Results and discussion
First, CeCl 3 solution was poured into NaOH solutions at different concentrations to prepare CeO 2 nanostructures. The XRD patterns (Fig. S2 †) show characteristic diffraction peaks for CeO 2 (JCPDS No. 34-0394). The relatively broad widths of the diffraction peaks, based on the Debye-Scherrer equation, suggests that the particle sizes should be relatively small. In addition, the widths of the diffraction peaks in XRD are similar for different samples, indicating that the sizes of different samples are similar. Transmission electron microscopy (TEM) was used to further characterize the CeO 2 nanoparticles (Fig. 1), revealing that morphologies of different products are quite different, despite the similar sizes based on the XRD data. CeO 2 samples generated with a low NaOH concentration of 0.1 to 1 mol L −1 , are mainly irregular particles with sizes ranging from 30 to 50 nm ( Fig. 1a and b). With increasing NaOH concentration (3 to 5 mol L −1 ), the products become rod-like, however, there is still a considerable proportion of nanoparticles in the product ( Fig. 1c-e). When the concentration of the NaOH solution reaches 6 mol L −1 , nanorods dominate with an average diameter of 10 nm, while a small number of nanoparticles also exists. These results indicate that a more concentrated alkali solution leads to more rapid dissolution/recrystallization rate of Ce(OH) 3 , and thus the rod-like morphology of CeO 2 . 28 To investigate the effects of hydrothermal temperature on the morphology, the alkali solution with a concentration of 6 mol L −1 was used, and three hydrothermal temperatures (100, 140 and 180°C) were employed to prepare CeO 2 nanostructures. The XRD patterns (Fig. S3 †) conrm that the obtained products are pure CeO 2 . Despite the similarity of the XRD patterns, the TEM images of the three products are very different (Fig. 2). At a low hydrothermal temperature of 100°C, nanorods with a diameter of 10 nm and some nanoparticles are obtained, while at a higher hydrothermal temperature of 140°C, many nanocubes with a length of 25-40 nm show up in the products, along with fewer nanorods. Most of products are nanocubes with a slightly larger size of 25-50 nm at a hydrothermal temperature of 180°C, and the HRTEM image show that these nanocubes mainly exposes (100) surface. Therefore, a low hydrothermal temperature of 100°C is found to be very important to obtain nanorods exposing (111) facets.
Next, hydrothermal treatment time was further optimized at a hydrothermal temperature of 100°C using the alkali solution with a concentration of 6 mol L −1 . Again, similar XRD patterns are observed for the products obtained with different hydrothermal times of 12, 24 and 36 h, conrming the formation of CeO 2 phases (Fig. S4 †). With a hydrothermal treatment time of 12 h, the product contains both nanorods and nanoparticles (Fig. 3a). With a longer hydrothermal time of 24 h, the amount of nanoparticles decreases and nanorods dominate, while the products are similar at a longer hydrothermal time of 36 h ( Fig. 3b and c). These results suggest that a relatively long hydrothermal time of 24 h is required for the nanorods to form and thus 24 h is chosen as the optimized hydrothermal time.
It has been shown that the morphology of CeO 2 nanoparticles may also be related to the Ce salts used present in the synthesis, 29 therefore, different cerium sources were also tested toward the synthesis of CeO 2 nanorods. Three common cerium compounds, including Ce(NH 4 ) 2 (NO 3 ) 6 , CeCl 3 and Ce(NO 3 ) 3 , which have different oxidation states for Ce, were selected for the synthesis. The XRD patterns (Fig. S5 †) show that CeO 2 phases can be obtained in all three cases. However, different morphologies are observed for the products. The TEM images show that nanorods dominate in the products, when using Ce(NO 3 ) 3 or CeCl 3 as the cerium source ( Fig. 4a and b). The CeO 2 nanorods obtained have a diameter of approximately 10 nm in both cases, while the length of the nanorods is 100-200 nm with Ce(NO 3 ) 3 as the source and the length decreases to 20-50 nm if CeCl 3 is used. However, when using Ce(NO 3 ) 3 as the cerium source, a small amount of nanocubes is attached to the nanorods, as shown by HRTEM (Fig. 4d). Because NO 3 − is more inclined to be adsorbed on (100) surface, it is conducive to the growth of nanocubes. 29,30 In contrast, larger nanosheets are  produced when Ce(NH 4 ) 2 (NO 3 ) 6 is used as the starting material (Fig. 4c). The differences observed in the products by using Ce 3+ or Ce 4+ salts as the source should be related to the formation mechanism of CeO 2 . Ce(OH) 3 is rst formed rapidly with Ce 3+ salts as the starting materials, while it is further oxidized to form CeO 2 , which is expected to be slower. No oxidation process is required if Ce(NH 4 ) 2 (NO 3 ) 6 is used as the cerium source, leading to a much faster process. 31 Therefore, the slow oxidation in the hydrothermal process is expected to play a key role in the formation of CeO 2 in a nanorod morphology.
Ce 3+ can be oxidized by a variety of methods, such as roasting oxidation, electrolytic oxidation, chemical oxidation and gas oxidation (such as oxygen gas or air). 32 Here we investigated the effects using different chemical reagents or oxygen gas concentrations. The XRD patterns of the corresponding samples are shown in Fig. S6, † conrming the formation of CeO 2 phases in all four cases. When H 2 O 2 was used as the chemical oxidant, it was added to the suspension generated aer quickly mixing CeCl 3 and NaOH solution. Granular and relatively uniform nanoparticles with small particle sizes of approximately 15 nm are obtained (Fig. 5a). In order to reduce  the oxidation rate, air was used as the oxidant, while a constant-ow pump was used to slowly drop the CeCl 3 solution into the NaOH solution under vigorous stirring, in order to control the formation rate of Ce(OH) 3 as well as oxidation, and the process was completed in 120 min. Again, the obtained products are granular and do not have specic shapes (Fig. 5b). Sample agglomeration also occurs, forming relatively large particles with a diameter of more than 50 nm, indicating that the reaction time for generating the precursor (Ce(OH) 3 ) as well as the oxidation process is too long. By increasing the rate for dropping CeCl 3 solution, the total reaction time can be decreased to 30 min, resulting in nanorod products with a small number of particles (Fig. 5c). By applying N 2 atmosphere protection and xing the reaction time to 30 min, the obtained samples are all rod-shaped (Fig. 5d). The average diameter is as small as 10 nm and the length is about 100-200 nm, which is associated with a large specic surface area (115 m 2 g −1 ). The results suggest that very low oxygen pressure (trace oxygen in nitrogen environment) is one of the keys in preparing CeO 2 nanorods. 33 The reaction time for forming the precursor (Ce(OH) 3 ), on the other hand, should be limited (i.e., adding CeCl 3 solution to NaOH solution within 30 minutes) in order to avoid agglomeration.
The HRTEM image of the same sample shown in Fig. 5d exhibits that the exposed surface of is (111), the most energetically favorable facet for CeO 2 (Fig. 5e). This nanorod sample with the most uniform morphology exposing (111) facet is named as CeO 2 -NR(111).
Finally, we used solid-state NMR spectroscopy to study the local environments of oxygen in the CeO 2 -NR(111) sample, and compared the results to the spectrum of CeO 2 nanorods prepared by literature method. To facilitate the comparison, the 17 O NMR spectrum of commercially available micron-sized ceria was collected (Fig. 6c), 26 which shows a sharp peak at 877 ppm, arising from the 4-coordinated oxygen ions (OCe 4 ) in the bulk part of ceria. The 17 O MAS NMR spectrum of CeO 2 -NR(111) enriched with 17 O 2 at 300°C (Fig. 6a) shows four peaks at 1033, 920, 825 and 877 ppm, which can be assigned to the oxygen ions at the rst, second, third layers of ceria (111) facets, and OCe 4 in the bulk part of the nanostructure, respectively, according to the previous work, 26 which conrms the successful preparation of CeO 2 nanorods preferentially exposing (111) facets. CeO 2 nanorods synthesized according to the method used in our previous 17 O NMR paper (CeO 2 -L) show four peaks at 1033, 920, 825 and 877 ppm in the 17 O MAS NMR spectrum (Fig. 6b) and the frequencies are similar to CeO 2 -NR(111). 20,34 However, the peaks of CeO 2 -L are much broader than CeO 2 -NR(111) ( Table 1), indicating that CeO 2 -NR(111) has a more ordered surface structure. 35 Therefore, CeO 2 -NR(111) is a better material suitable for 17 O NMR studies of ceria nanorods. It is worth mentioning that the additional peaks at 1033, 920 and 825 ppm due to oxygen ions at rst, second and third layers are not observed in micron-sized ceria (Fig. 6c), which can be ascribed to the very small surface area and the corresponding low concentrations of these surface sites in this sample.

Conclusion
In this study, we explored the hydrothermal synthesis conditions for preparing CeO 2 nanorods that exposes (111) facets with a well-dened structure suitable for 17 O solid state NMR investigations. Several important factors affecting the morphology of CeO 2 nanostructures were tested, including alkali solution concentration, hydrothermal temperature and time, cerium source and oxidation agent. The CeO 2 nanorods prepared with the optimized conditions exhibit much narrower signals in the 17 O MAS NMR spectrum, compared to the those prepared using the method from previous work. This study provides a foundation for future investigations into detailed mechanisms of reactions involving CeO 2 nanorods (111) using 17 O solid state NMR spectroscopy.

Author contributions
C. Yang, L. Peng and W. Ding, contributed the design of the experiment and writing of the paper. X. Ning, X. Hou, and X. Xia carried out characterization of the samples. S. Chen and Z. Zhang took the TEM and HRTEM images of the samples. All the authors have given their approval to the nal version of the manuscript.